An atom probe (e.g., atom probe microscope) is a device which allows specimens to be analyzed on an atomic level. For example, a typical atom probe includes a specimen mount, a counter or local electrode, and a detector. During analysis, a specimen is carried by the specimen mount and a positive electrical charge (e.g., a baseline voltage) is applied to the specimen. The detector is spaced apart from the specimen and is either grounded or negatively charged. The counter electrode is located between the specimen and the detector, and is either grounded or negatively charged. A positive electrical pulse (above the baseline voltage) and/or a laser pulse (e.g., photonic energy) are intermittently applied to the specimen. Alternately, a negative voltage pulse can be applied to the electrode. Occasionally (e.g., one time in 10 to 100 pulses) a single atom is ionized near the tip of the specimen. The ionized atom(s) separate or “evaporate” from the surface, pass though an aperture in the electrode, and impact the surface of the detector, typically a micro channel plate (MCP). The elemental identity of an ionized atom can be determined by measuring its time of flight (TOF) from the surface of the specimen to the detector, which varies based on the mass-to-charge-state ratio (m/z) of the ionized atom. The location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Accordingly, as the specimen is evaporated, a three-dimensional map of the specimen's constituents can be constructed. While the process is considered a point-projection with extremely high magnification (approximately×1 million) the resultant data can be analyzed in virtually any orientation, hence can be considered more tomographic in origin.
Difficulties associated with atom probe tomography (APT) include but are not limited to detector efficiency, trajectory aberrations, non-uniform magnification, limited or non-existent a priori information regarding compositions and interface morphologies and the like.
Since the atom probe process is destructive the dynamics of the specimen evaporation and erosion process complicate device operation issues as well as data reconstruction. If a specimen includes multiple layers of different atomic species, then the induced fields can result in either preferential evaporation in specific regions or specimen fracture. Further, as the specimen tip erodes, the evolution of the tip shape further complicates both the control of the field magnitude as well as the reconstruction of the resultant data.
Atom probe data reconstruction problems are exacerbated by all of the aforementioned effects. Even if the typical detection efficiency of around 50% was somehow improved to >95% the other aberrations and magnification errors could seriously degrade the resultant data. As such, a number of corrections are made to the raw and intermediate data in order to obtain useful output data. A full discussion of the issues can be found in Atom Probe Tomography: Analysis at the Atomic Level by M. K. Miller, Kluwer Academic/Plenum Press (2000), which is incorporated herein by reference.
Transmission electron microscopy (TEM) is a technique wherein electrons are transmitted through a very thin specimen and the interactions between the electrons and the specimen are detected by various means. TEM images can resolve individual atoms but are considered projection by nature. It is typically non-destructive and can be performed on specimens prior to the destructive atom probe procedure, thus creating two unique and complementary data sets from the same specimen. For a discussion of TEM refer to Transmission Electron Microscopy, D. B. Williams and C. B. Carter, Plenum Press, NY (2006), which is incorporated herein by reference.
Scanning transmission electron microscopy (STEM) differs from TEM in that the electron beam is swept across the specimen, rather than held in one place. Other techniques include high resolution transmission electron microscopy (HRTEM) and a host of other variants. As used herein, the acronym TEM or TEM/STEM generally refers to all of these techniques unless specifically noted.